Unsaturated Flow Drying Beds effectively treat septic sludge from septic tanks. However, the sustainability of this purification process depends on the organic load applied and the availability of oxygen. To guarantee this sustainability, a flow model in an unsaturated medium coupled with the variation of the biofilm was developed. The calibration, validation, and comparison of experimental and simulation results were carried out on the basis of the NASH criterion. The experimental results show that for the organic load of 34 mgO2/cm2/d, when the sludge with a concentration of 2537 mgO2/l is applied to the hydraulic load 4.28 cm/d, the purification efficiency is 95%. On the other hand, for the same organic load, when the sludge with a concentration of 1175 mgO2/l is applied to the hydraulic load of 10 cm/d, the purification efficiency is 79%. The simulation results confirm this paradox of purification yields. They reveal that for a sludge with a concentration of 2537 mgO2/l applied to a hydraulic load of 4.28 cm/d, most of the pollution is retained in the first four centimeters of the filter beds with a residence time of 6h26mn. On the other hand, for a sludge with a concentration of 1175 mgO2/l applied to a hydraulic load of 10 cm/d, the pollution goes down to a depth of 13 cm with a residence time of 4h21mn. Therefore, unsaturated flow drying bed technology makes it possible to easily treat heavily loaded fecal sludge at low hydraulic loads. Which thus guarantees good purifying efficiency and slows down clogging.
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